1. Field of the Invention
The present invention relates to improved thermoelectrics for producing heat and/or cold conditions with greater efficiency through operation in a non-equilibrium condition.
2. Description of the Related Art
Thermoelectric devices (TEs) utilize the property of certain materials to develop a thermal gradient across the material in the presence of current flow. Conventional thermoelectric devices utilize P-type and N-type semiconductors as thermoelectric material within the device. These are physically and electrically configured in such a manner that the desired function of heating or cooling is provided.
Several configurations for thermoelectric devices are in current use for automobile seat cooling systems, for portable coolers and refrigerators, for dispensing systems, for scientific applications, for cooling electronics and fiber optic systems, for cooling infrared systems and many other uses. However, Conventional TEs have many sources of inefficiency, and the current efficiency levels of conventional TEs limits their practical applications.
Some fundamental equations, theories, studies, test methods and data related to TEs for cooling and heating are described in Angrist, Stanley W., Direct Energy Conversion, 3d Edition, Allyn and Bacon, Inc., Boston, Mass. (1976). The most common configuration used in thermoelectric devices today is illustrated in FIG. 1A. Generally, P-type and N-type thermoelectric elements 12 are sandwiched in an assembly 10 between two substrates 14. The thermoelectric elements 12 are connected in series via copper shunts 16, soldered to the end of the elements 12. A current, I, passes through both element types. A DC voltage 18, when applied, creates a temperature gradient across the TE elements.
As described in further detail in this description, the losses in a thermoelectric (TE) device can be reduced by operating the TE in a non-equilibrium condition. The specific operation can be understood by referring to FIG. 1B, which shows the evolution of the temperature profile 100 of a TE element with a cold side 101 at O and a hot side 106 at L. The hot side 106 is a heat sink and does not change temperature with time. The cold side""s 101 temperature decreases with time, and eventually reaches the time independent equilibrium profile 104 at ts and cold side 101 temperature TCS. The cool side""s 101 equilibrium temperature TCS depends in part on the amount of thermal power being transferred at the cold side 101 to cool a working fluid or an object placed at that location.
The temperature TA shows the TE element at uniform temperature before current is applied. The temperature profile 102, at time t1, depicts the temperature distribution shortly after current is first applied, but well before equilibrium is established at time ts. At time t1, the cold side 101 temperature is TC1. At point X=X1, the temperature moves above ambient, TA, due to Joule heating within the TE element. At X=L, the hot side 106 temperature is TA due to the imposed heat sink condition. Similarly, at a slightly later time, t2, the cold side 101 temperature TC2, is lower and the position, X, within the TE at which the TE element is at temperature TA has moved to the left to X=X2.
Until the time t5, a portion of the Joule heating in the element is conducted to the heat sink at the hot end 106. At t5, that effect is zero. For slightly longer times, heat will be conducted from the heat sink 106 into the TE element. Thus, up to t5, no heat enters the TE element from the hot end 106 heat sink, so that all the cooling at X=0, is conserved within the element. In addition, some of the losses from Joule heating have been transported out the hot side 106, thereby slightly reducing the amount of Joule heating of the TE element.
These combined effects can be used to reduce cooling loss from the TE element and thereby provide a net benefit. Similar benefits are achievable in the heating mode and with TE systems that have separate sections to cool and heat working fluids.
Representative designs that utilize this effect are described in the figures that follow. These designs and related designs that exhibit the same or similar performance are also a part of the present invention.
A thermoelectric system is disclosed that has a plurality of thermoelectric elements forming at least one thermoelectric array with at least one first side and at least one second side exhibiting at least one temperature gradient between them during operation. The thermoelectric elements are in thermal communication with at least one heat exchanger on at least the first or the second side, wherein at least a portion of the thermoelectric array is configured to be operated between or among at least first and second different current levels, wherein at least the first current level is below a current level that provides substantially maximum steady-state TE cooling or heating. In one embodiment, the operation between at least first and second current levels is substantially transient or non-equilibrium at each level. Preferably, at least some of the thermoelectric elements exchange heat with at least one working fluid while such thermoelectric elements operate at the first current level and/or while such thermoelectric elements operate at the second current level. Advantageously, the first current level is substantially below a current level that provides substantially maximum steady-state TE cooling or heating, and in one embodiment, is substantially zero. In one embodiment, the second current level is below, at or above a current level that provides substantially maximal steady-state TE cooling or heating.
The current level may be varied among at least the first and the second levels, varied between these levels, and/or varied among or between programmed levels. The current levels may also be varied to provide a cyclic pattern, such as a sine wave, or the like. The levels are preferably selected to provide improved efficiency over steady-state operation.
In one described example, the at least one thermoelectric array is configured to move such that at least some of the thermoelectric elements couple to at least one power source for a predefined period of time, and decouple from the at least one power source for a predefined period of time. This configuration may provide for at least some of the thermoelectric elements exchanging heat with at least one working fluid while decoupled from the power, and/or while coupled to the power.
An additional enhancement can be obtained by configuring the thermoelectric elements such that their active circuit or resistance is adjusted. In one described embodiment, the thermoelectric array is configured in a generally circular configuration, and is configured to rotate in a first direction about an axis of rotation, and at least one working fluid travels along at least one heat exchanger in a direction opposite of the direction of rotation. Similarly, the at least one working fluid may travel along the at least one heat exchanger in the first direction. The thermoelectric may be used for cooling, heating, or both cooling and heating.
A method of improving efficiency in a thermoelectric system having a plurality of thermoelectric elements is also disclosed. The method involves forming at least one thermoelectric array having at least one first side and at least one second side exhibiting at least one temperature gradient between them during operation. Power is applied to at least some of the thermoelectric elements in the at least one thermoelectric array in a non-steady state manner to operate the at least some of the thermoelectric elements among or between at least first and second different current levels, wherein at least the first current level is below a current level that provides substantially maximum steady-state TE cooling or heating.
In one embodiment, at least some of the thermoelectric elements exchange heat with at least one working fluid. At least some of the thermoelectric elements may exchange heat with at least one working fluid while such thermoelectric elements operate at the first current level and/or while such thermoelectric elements operate at the second current level. Preferably, the first current level is substantially below a current level that provides substantially maximum steady-state TE cooling or heating. In one embodiment, the first current level is substantially zero.
The current levels may be preprogrammed, variable, or otherwise applied. The current may be applied in a cyclic pattern as well. In one example, the step of applying power to at least some of the thermoelectric elements involves coupling the thermoelectric elements to a power source for a predefined period of time, and disconnecting these thermoelectric elements for a predefined period of time.
An additional step involves adjusting the resistance of at least some of the thermoelectric elements.